Environmental Engineering Reference
In-Depth Information
V D ,y D
x F =0.03
y n
x n -1
q =1:feed is saturated liquid
n th stage
at the boiling point.
S = V D :equimolar overflow
95% of alcohol in feed is in V D .
B, x B
S
, y =0
S
Figure 4.17 Stripping tower with direct steam injection (dotted line indicates
control volume), Example 4.5.
The overall mass balance on the lower section is:
+ S
S
L
=
V
+
B
L
V
=
B
.
(4.35)
The intersection of the operating line with the diagonal ( y
=
x line):
Bx B
B
x
=
(not x B )
.
S
Bx B
L
=
=
The intersection with the x -axis ( y
0):
x
.
But when assuming constant molar overflow:
S
=
V
B
=
L
x
=
x B
when
y
=
0
.
Stepping off stages: use ( x B ,0)asanend point of operating line.
Example 4.5: stripping tower and direct steam injection
Problem:
A liquid ethanol-water feed at the boiling point contains 3 mol% ethanol and enters
the top tray of a stripping tower, as shown in Figure 4.17. Saturated steam is injected
directly into the liquid in the bottom of the tower. 95% of the alcohol in the feed
exits in the overhead. Assume equimolar overflow for this problem and an equilibrium
relationship:
y
=
8
.
3 x
.
(a) Calculate the minimum moles of steam needed per mole of feed.
(b) Using 2.5 times the minimum moles of steam, calculate the number of theoretical
stages needed and the composition of the overhead and the bottoms streams.
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